Structure of K₂TaF₇ at 993 K: the combined use of synchrotron powder data and solid-state DFT calculations

The structure of dipotassium tantalum heptafluoride, K₂TaF₇, was optimized by energy minimization in the solid state using a plane-waves DFT (density functional theory) computation for which the lattice parameters were obtained by the Le Bail technique from synchrotron X-ray powder diffraction data collected at 993 K. Owing to the sample's corrosiveness, it had to be loaded in a thin-walled Pt capillary. It was found that the structure corresponds to that of the β-K₂TaF₇ phase. The Ta atoms in the TaF₇⁻ polyhedra are sevenfold coordinated by fluorine atoms positioned within 1.977–2.007 Å. The K atoms are surrounded by eleven (K1) or eight (K2) fluorine atoms. Every F atom in the structure is surrounded by three K atoms. The F...K contact distances vary from 2.57 to 3.32 Å. It is suggested that solid-state DFT methods may, in many cases, provide an alternative tool to standard Rietveld refinements for structure determination.

Computing details

Program(s) used to refine structure: FULLPROF; molecular graphics: Diamond; software used to prepare material for publication: PLATON.

Figures

Dipotassium heptafluorotantalate(V)

Crystal data

F7Ta·2(K)	V = 727.84 (9) Å^3
Mr = 392.15	Z = 4
Orthorhombic, PNMA	X-ray radiation, λ = 0.398230 Å
Hall symbol: -P 2ac 2n	T = 993 K
a = 10.0731 (7) Å	white
b = 5.9456 (4) Å	cylinder, ? × ? × ? mm
c = 12.1527 (9) Å

Data collection

'ESRF, ID31' diffractometer	Data collection mode: transmission
Radiation source: synchrotron	2θmin = 4.006°, 2θmax = 14.989°, 2θstep = 0.009°
Specimen mounting: Pt capillary

Refinement

Rp = 9.646	Profile function: Lorentzian
Rwp = 13.361	10 parameters
Rexp = 5.579	0 restraints
RBragg = 18.477
χ^2 = NOT FOUND	Background function: linear approximation
1203 data points	Preferred orientation correction: none

Special details

Experimental. The powder was loaded into a X·X mm Pt capillary with wall thickness of Y·Y mm. The sample was heated by a hot air blower mounted perpendicular with respect to the capillary.

Refinement. The structure was refined by geometry optimization done by cell energy minimization. This is why no estimated standard deviations are given for atomic coordinates.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
ta1	0.24260	0.25000	0.13150	0.074 (4)*
k1	0.12810	0.75000	0.95060	0.25 (2)*
k2	0.46380	0.25000	0.82820	0.25 (2)*
f1	0.35950	0.03360	0.05480	0.147 (17)*
f2	0.09270	0.03340	0.14590	0.147 (17)*
f3	0.30000	0.04770	0.25230	0.147 (17)*
f4	0.16340	0.25000	0.97990	0.147 (17)*